There is known a pneumatic tire having an innerliner disposed facing the cavity of the tire as an air impermeable layer, the innerliner is formed of a film of a thermoplastic resin or a thermoplastic elastomer composition having a thermoplastic resin and an elastomer blended therewith instead of rubber in order to reduce the tire weight (see a patent document 1, for example).
A pneumatic tire having an innerliner formed of such a film employs a method including a step of forming an innerliner on an expandable and contractible building drum of a tire building machine by disposing a film preformed of a cylinder around the building drum (see a patent document 2, for example). Since the film is preliminarily formed as a cylinder, the film can be maintained on the expanded building drum using a contractive force of the film. Therefore, there is an advantage that complication of the structure of the tire building machine can be avoided.
More specifically, when a sheet-shaped film is used in the alternative of the cylinder film, a suction structure for suctioning and holding the sheet-shaped film that has been wrapped around the building drum is required. In order to securely hold the sheet-shaped film, it is necessary to provide a lot of suction openings in the entire outer circumferential surface of the building drum, suction passages communicating with the suction openings and the like, which makes the structure of the tire building drum complicated. However, when the cylinder film is used, the suction structure is not required.
When a structure which suctions and holds only the wrapping-starting end and wrapping-finishing end of the sheet-shaped film is employed in order to avoid complicating the structure of the tire building drum, ability for holding the film is significantly reduced. As a result, when another tire component is applied onto the film, the film moves easily, which produces a problem of significantly degrading accuracy of tire component application.
However, when the innerliner is formed on the building drum using the above cylinder film, the following problem is created. That is, a carcass ply is further applied onto the film via a rubber layer (a rubber layer for obtaining adhesion to the carcass ply). Since the building drum having an expandable and contractible construction comprises a plurality of drum pieces which are jointed together, depressions such as joint lines, screw holes and the like exist in the outer circumferential surface of the building drum on which the film is held. When the rubber layer has been applied onto the film, the film is partially depressed into the depressions because the film is thin, which creates a problem of being prone to form air pockets between the rubber layer and the film at locations of the depressions. If the air pockets are formed, trouble which occurs is that the inner surface of a tire is partially ballooned at locations of the air pockets after vulcanization.
Patent Document 1: Japanese Patent Application Kokai Publication HEI 10-35232
Patent Document 2: Japanese Patent Application Kokai Publication 2002-18973